04 September 2008

Which animal phylum consists of one and only one species? The Placozoa. And the only species in that phylum is the simplest animal of them all, Trichoplax adhaerens.

I have never seen a Trichoplax, which is a rather small (1-2 mm) creature. But I have seen members of smaller phyla, such as rotifers. So size is not a reason why I have not seen a Trichoplax. In fact, some other phyla that have members much larger than Trichoplax are among those phyla that I have so far not had the pleasure of getting acquainted with. What are those phyla?

There are some significant differences between these disparate lists. For example, the ADW list includes the Myxozoa, whereas the other 2 lists don't; the UCMP list unites the rotifers and the acanthocephalans, while the Wikipedia list keeps them separate. It is really pointless to attempt to consolidate them or to argue that one is better than the other.

Instead, I will list a compilation of some of the phyla of which I have yet to see a live member: Acanthocephala, Chaetognatha, Gnathostomulida, Hemichordata, Nematomorpha, Nemertea, Onychophora, Priapulida, Sipuncula, Echiura, Phoronida, Loricifera, Pogonophora, Brachiopoda, Placozoa and so on.

I may have seen a kinorhynch once while looking at a marine sediment sample under a microscope, but I lost the animal—whatever it was—after a brief glance at it. Also, I am not sure if I have seen bryozoans.

I would like to see at least one live member of all animal phyla before I die.